IDAHO'S FIRST DINOSAUR TRACKS AND A U-PB DETRITAL ZIRCON AGE FOR THE EARLY CRETACEOUS DRANEY LIMESTONE

The Gannet Group preserves 5000 feet of sediment deposited primarily in the Early Cretaceous foredeep depocenter of Idaho. Included in the upper portion of this group is the Draney Limestone which consists of interbedded shales and limestones deposited in one or more ephemeral lakes. As shorelines fluctuated within the lakes, limey muds were exposed along the shoreline and trampled by dinosaurs. Due to the softness of these muds plus extensive trampling and other bioturbation, these tracks were not optimally preserved. We tentatively identify the trackmakers as bipedal ornithischians, possibly iguanodontids, but there may be more than one taxon of trackmaker. Freshwater fish, hybodontid shark, turtle, and invertebrate fossils were preserved in the same sediments as the tracks and in sediments above and below the track horizon. Based on microfossils, the age of the formation is generally listed as Albian. We recovered apatite and zircon crystals from a shale unit associated with the dinosaur track horizons in exposures in Tincup Canyon, Idaho. The euhedral nature of these crystals suggests that they represent a volcanic ash fall which was only slightly reworked. The zircons were analyzed via laser ablation multi-collector inductively coupled plasma mass spectrometry which provided a single crystal U-Pb age of 112.5 ⁺2.6 ^-2.7 Ma based on a coherent group of 20 crystals using the TuffZirc routine. This coincides with the Aptian-Albian stage boundary and represents the first radiometric age for the Draney Limestone.